MAFIC ROCKS of the PANTS LAKE INTRUSION and RELATED Ni–Cu–Co MINERALIZATION in NORTH–CENTRAL LABRADOR

Current Research (1999) Newfoundland Department of Mines and Energy Geological Survey, Report 99-1, pages 215-253 MAFIC ROCKS OF THE PANTS LAKE INTRUSION AND RELATED Ni–Cu–Co MINERALIZATION IN NORTH–CENTRAL LABRADOR A. Kerr Mineral Deposits Section ABSTRACT The Mesoproterozoic Pants Lake intrusion (PLI) was emplaced passively into sulphide-bearing paragneisses and orthogneisses of the Churchill Province, immediately west of the Nain–Churchill boundary. In the four years since the Voisey's Bay discovery, the PLI has become a major focus for Ni–Cu exploration, and is now known to host widespread disseminated, and rarer massive, magmatic sulphide mineralization. The PLI represents the closest analogue to the Voisey's Bay troctolitic host rocks recognized in northern Labrador. The three main units of the PLI are a fine-grained, variably layered olivine gabbro containing cumulus olivine, a massive, coarse-grained leucogabbro containing late interstitial olivine, and a black olivine gabbro that is texturally akin to the massive leucogabbro and locally difficult to discriminate from it. The coarse-grained unit appears to postdate the layered unit, but the relationship of the black olivine gabbro to other units is ambiguous. The PLI component intrusions are broadly sheet- like in geometry, and range in thickness from < 50 m to over 600 m. Their present geometry is complex, and may reflect the superposition of tilting and/or gentle folding on primary intrusive variations in thickness and anatomy. The mineralized basal contacts of intrusions lie within feasible exploration depths over large areas, and are far from fully explored. Magmatic sulphide mineralization is ubiquitous near the basal contacts of the PLI component intrusions, and is commonly associated with distinctive, contaminated, sulphide-bearing mafic rocks that include textural equivalents of the "basal breccias" and "leopard troctolites" described from the Voisey's Bay deposit. Metal contents at 100 percent sulphide are, on average, lower than at Voisey's Bay, but Co is significantly higher at a given Ni content. There are indications of higher metal contents, up to 11.8% Ni and 9.7% Cu, in sulphide zones that may represent fractionated sulphide liquids. The PLI mineralized sequences appear to be intrusion-wide "stratigraphic" features, rather than being confined to feeder conduit systems, and their distribution and relationships to the intrusions may differ from the Voisey's Bay area. In both areas, the mineralized sequences record the emplacement of discrete sulphide- and fragment-bearing magma pulses into active, dynamic, mafic magma chambers, but the precise details of their evolution may differ. However, the PLI clearly demonstrates that the distinctive rock types and processes seen (and inferred) at Voisey's Bay are present elsewhere in Labrador. INTRODUCTION and mafic host rocks since 1996. This article brings togeth- er observations and ideas developed over this period, and Mineral exploration in northern Labrador increased dra- integrates them with results from the exploration program. matically in 1995, following the Voisey's Bay discovery of Interpretations presented here are current (but still early) late 1994, but overall activity has declined since 1996. views, and will be refined and/or modified as additional However, the "South Voisey's Bay Project" of Donner information is gathered from continued exploration and Minerals and their joint venture partners progressively related research theses. However, in view of the wide inter- expanded from 1995 to 1998, reflecting positive indicators est in this project an overview of this type is timely. of economic-grade mineralization, and recognition that the geology has many intriguing similarities to the area around The study area is located in north–central Labrador, Voisey's Bay itself (e.g., Kerr and Smith, 1997; Fitzpatrick about 130 km south of Nain and 90 km west of Hopedale, et al., 1998). and forms part of a highland area north of the Adlatok (or Ugjoktok) River. Regionally, the area (Figure 1) is dominat- The Geological Survey of Newfoundland and Labrador ed by Paleoproterozoic gneisses of the Churchill Province, has been examining the regional geology, mineralization and is situated between two major Mesoproterozoic igneous 215 CURRENT RESEARCH, REPORT 99-1 Figure 1. General geology of northern Labrador, showing the Nain and Churchill provinces, and the Mesoproterozoic Nain and Harp Lake plutonic suites. The area of active exploration at the South Voisey's Bay project, and the Pants Lake intrusion, are indicated. 216 A. KERR complexes, i.e., the Nain Plutonic Suite (1350 to 1290 Ma) north to the Nain Plutonic Suite (although none are dated). and the Harp Lake Plutonic Suite (~ 1450 Ma). The Nain The youngest component of the latter is the Pants Lake Province–Churchill Province boundary zone lies in the east- intrusion consisting of an elongated belt of mafic plutons ern part of the area, but its characteristics in this region are running northwest–southeast through the centre of the area poorly known (Ryan, 1996). The gneisses of the Churchill (Figure 2). Province are intruded, over a wide area, by post-tectonic mafic plutonic rocks that collectively form the Pants Lake NAIN PROVINCE BASEMENT ROCKS intrusion (PLI), assigned to the Nain Plutonic Suite. The PLI hosts widespread Ni–Cu sulphide mineralization, and is the These are described by Thomas and Morrison (1991), main focus of this article. and were reexamined adjacent to the Nain–Churchill boundary. The rocks are dominated by banded, grey, tonalitic to Sulphide mineralization was first noted by Thomas and granodioritic orthogneiss; the banding partly reflects trans- Morrison (1991), who also initially delineated the mafic posed primary compositional variation and partly, the results rocks of the PLI. There was essentially no exploration prior of migmatization. Concordant bands of dark grey to black to late 1995, when Donner Resources acquired an interest in amphibolite represent either supracrustal remnants or trans- a large tract of land centred on anomalous Ni values in lake posed mafic dykes. No intense deformation or mylonitiza- sediment, which are spatially associated with the PLI mafic tion were observed in Archean gneisses adjacent to the rocks. Public-domain information released from the explo- Nain–Churchill boundary, but it is present in paragneiss ration program has been summarized in previous reports west of the boundary (see below). The Archean gneisses (Kerr and Smith, 1997, 1998; Kerr, 1998a), and is not contain small shear zones adjacent to, and subparallel with, repeated here. The 1998 program was the most extensive to the boundary. date, and included about 15 000 m of diamond drilling. Although no economic deposits have been discovered, CHURCHILL PROVINCE BASEMENT ROCKS Donner Minerals have stated their intention to mount a 1999 exploration program. Pelitic and Psammitic Paragneiss This is the dominant unit within the Churchill Province REGIONAL GEOLOGY in the study area, as noted by Thomas and Morrison (1991). Subsequent company mapping has confirmed this pattern, The geology of the area is depicted in Figure 2. The but the gneisses are highly variable on an outcrop scale, and present outline and subdivision of the PLI is mainly derived mappable compositional variants are difficult to define. from mapping by Donner Minerals and Teck Corporation Paragneiss outcrops invariably contain up to 50 percent leu- (Wares, 1997; Fitzpatrick et al., 1998), whereas country cocratic granitoid gneiss and pegmatite, and granitoid gneiss rock units are taken from Hill (1982), Thomas and Morrison outcrops invariably contain numerous screens and zones of (1991) and company mapping. The current project is aimed paragneiss. Thus, delineation of "paragneiss" and mostly at understanding mineralization, and no systematic "orthogneiss" units is inherently subjective. The pattern mapping was conducted. However, key contacts and sec- illustrated in Figure 2 is derived mainly from company map- tions were examined, and areas outside the joint venture ping (Fitzpatrick et al., 1998), and modified from Thomas were mapped at a reconnaissance level. and Morrison (1991) in areas outside the exploration project. GEOLOGICAL FRAMEWORK Paragneisses are white- to pale-grey- or rusty-weather- The area includes four main "packages" of rocks. In the ing, and well banded. Banding is defined primarily by the east, quartzofeldspathic orthogneisses and associated alternation of granitic layers and biotite-rich bands, and like- supracrustal rocks are part of the Hopedale Block of the ly records repeated migmatization, and deformation. Well- Archean Nain Province. These are in presumed tectonic banded gneiss commonly passes along strike into nebulitic contact with gneisses and granitoid rocks of the Churchill material indicating, some late, static migmatization. The Province, which are dominated in this area by darker, restitic material consists of quartz, plagioclase, K- Paleoproterozoic supracrustal rocks. These are dominated feldspar, biotite, muscovite, garnet and sillimanite, indicat- by pelitic to psammitic paragneiss and leucocratic granitic ing amphibolite-facies conditions. Cordierite is locally orthogneiss, commonly intermixed on an outcrop scale. prominent, and likely present as a cryptic phase in most out- Undeformed Mesoproterozoic anorthosites, diorites and crops. Leucocratic zones commonly contain large garnets up granites occur in the north and

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